Treatments that restore p53 activity could provide a breakthrough in cancer therapy given that p53 is mutated in many cancers. Successful targeting of p53 is an unmet clinical need evidenced by the lack of FDA-approved drugs in this category. We have collaboratively developed and tested ReACp53, a peptide that re-enables p53 function by preventing its aggregation. Our previous work demonstrated the efficacy of ReACp53 monotherapy in targeting high-grade serous ovarian cancers (HGSOCs), which are tumors characterized by loss of p53 function. HGSOCs are deadly gynecologic malignancies that claim the lives of 14,000 U.S. women annually despite radical surgeries and administration of carboplatin standard chemotherapy. We hypothesize that ReACp53 can sensitize platinum-resistant HGSOCs to carboplatin. This combination therapy may prevent and target recurrence of this aggressive cancer. Building on our previous work, we propose the following aims that will facilitate clinical translation of our therapeutic: (1) Determine if platinum-resistant ovarian cancer cells can be sensitized to carboplatin when treated with ReACp53. A novel in vitro 3D miniring organoid drug assay will be used to test the sensitivity of 72 primary HGSOCs in this aim. Subaim 1.1 will focus on testing this therapy in targeting newly-diagnosed HGSOCs enriched for platinum-resistant cells following exposure to neoadjuvant carboplatin. This response will be compared to matched pre-therapy tumor specimens from the same patient. In Subaim 1.2 we will test the efficacy of this combination in targeting recurrent platinum-resistant HGSOCs. p53 mutation status in all specimens will be correlated with drug response. Results will demonstrate if this therapeutic approach can be efficacious in targeting platinum-resistant HGSOCs. (2) Determine the effect of p53 mutations on modulating ReACp53 and carboplatin response using in vitro and in vivo tumor models. Here, we will take a genetic approach by engineering platinum-resistant p53-null ovarian tumor cells to express known and common p53 mutations reported in HGSOCs. Response of these tumor cells to combination therapy will be tested in vitro in Subaim 2.1 and in vivo in Subaim 2.2. Results will demonstrate if the addition of ReACp53 to carboplatin can potentiate cell death in platinum-resistant ovarian cancer cells expressing common p53 mutations found in up to 30% of all HGSOCs. (3) Test the efficacy of ReACp53 and carboplatin combination therapy in targeting platinum-resistant HGSOC PDXs. p53 mutations are highly implicated in mediating platinum resistance in many carcinomas. Here, we will test if the addition of ReACp53 to carboplatin can restore platinum sensitivity of recurrent human HGSOCs using PDX models. In Subaim 3.1 PDXs will be established from platinum resistant HGSOCs, 3 sensitive and 3 resistant to combination therapy based on in vitro drug testing. In Subaim 3.2, mice bearing PDXs will be treated with vehicle, ReACp53, carboplatin, or the combination therapy. In vivo tumor response will be correlated with (a) in vitro drug sensitivity and (b) p53 mutation status. Findings may provide the rationale and tools for selecting tumors that may benefit from this therapeutic approach in future clinical trials. Collectively, by restoring p53 function and defining biomarkers of response to this therapy, we may be able to define a new and more effective precision medicine therapeutic approach for Veteran patients diagnosed with p53-driven ovarian cancers. The potential impact of our work is broad and may extend beyond female Veterans as this therapy can be applicable to the 20,000 female and male Veterans diagnosed with aggressive p53- mutated tumors each year.